Even with the best intentions, life happens, and sometimes immigration documents expire by mistake. If this happens to you, you should still file Form I-90 to renew your green card, just like those waiting for citizenship. Explain any reasons for the delay in your application. Late fees will likely apply.
If your green card expires, contact USCIS to inform them about the mistake and your pending citizenship case. Ask if you qualify for a “good cause” waiver due to your specific circumstances. Continue to pursue your citizenship application and interview. Once approved, report the end of your permanent residency status to USCIS with your citizenship certificate in hand.
If you are interviewed by border or immigration officials after your green card has expired, remain respectful and explain you have filed to renew your expired green card due to a pending citizenship case. Carry copies of your paperwork with you to help minimize problems.